Lang reportedly will receive $28.5 million with $19 million guaranteed.
Lang spent his first eight NFL seasons with the Green Bay Packers was a Pro Bowl selection last season for the first time.
He chose the Lions over the Seattle Seahawks and Packers.
Lang said during the process that where to raise his family would be part of his decision process. That apparently was a plus for the Lions as Lang grew up in the suburbs of Detroit and played college football at Eastern Michigan.
Lang was a fourth-round selection in 2009 and played 119 games (94 starts) for the Packers. He played 13 games last season, missing three with a foot injury.
The 29-year-old underwent offseason hip surgery and is expected to be recovered by the start of training camp.
Lang is the second offensive lineman to leave the Packers during free agency. Center JC Tretter departed to sign a three-year contract with the Cleveland Browns.